Hemolytic disease of the fetus and newborn: systematic literature review of the antenatal landscape

Background Prevention of pregnancy-related alloimmunization and the management of hemolytic disease of the fetus and newborn (HDFN) has significantly improved over the past decades. Considering improvements in HDFN care, the objectives of this systematic literature review were to assess the prenatal treatment landscape and outcomes of Rh(D)- and K-mediated HDFN in mothers and fetuses, to identify the burden of disease, to identify evidence gaps in the literature, and to provide recommendations for future research. Methods We performed a systematic search on MEDLINE, EMBASE and clinicaltrials.gov. Observational studies, trials, modelling studies, systematic reviews of cohort studies, and case reports and series of women and/or their fetus with HDFN caused by Rhesus (Rh)D or Kell alloimmunization. Extracted data included prevalence; treatment patterns; clinical outcomes; treatment efficacy; and mortality. Results We identified 2,541 articles. After excluding 2,482 articles and adding 1 article from screening systematic reviews, 60 articles were selected. Most abstracted data were from case reports and case series. Prevalence was 0.047% and 0.006% for Rh(D)- and K-mediated HDFN, respectively. Most commonly reported antenatal treatment was intrauterine transfusion (IUT; median frequency [interquartile range]: 13.0% [7.2–66.0]). Average gestational age at first IUT ranged between 25 and 27 weeks. weeks. This timing is early and carries risks, which were observed in outcomes associated with IUTs. The rate of hydrops fetalis among pregnancies with Rh(D)-mediated HDFN treated with IUT was 14.8% (range, 0–50%) and 39.2% in K-mediated HDFN. Overall mean ± SD fetal mortality rate that was found to be 19.8%±29.4% across 19 studies. Mean gestational age at birth ranged between 34 and 36 weeks. Conclusion These findings corroborate the rareness of HDFN and frequently needed intrauterine transfusion with inherent risks, and most births occur at a late preterm gestational age. We identified several evidence gaps providing opportunities for future studies. Supplementary Information The online version contains supplementary material available at 10.1186/s12884-022-05329-z.

Page 2 of 10 de Winter et al. BMC Pregnancy and Childbirth (2023) 23:12 Background Despite advances in the prevention of pregnancy-related red blood cell immunization and management and treatment of pregnancies affected by hemolytic disease of the fetus and newborn (HDFN) over recent decades, the disease still poses a significant risk in affected pregnancies [1,2]. HDFN is caused by maternal alloimmunization through exposure to incompatible red blood cell antigens of the fetus or through incompatible blood transfusion [1,3]. The then-formed immunoglobulin G (IgG) antibodies are actively transported across the placenta and can cause fetal hemolysis and anemia. When untreated, progressive fetal anemia results in hydrops fetalis and ultimately fetal demise. If the fetus survives, persistent hemolysis causes neonatal anemia and hyperbilirubinemia, which-when untreated-ultimately leads to a severe cerebral condition ("kernicterus"). No cure exists for HDFN. Hence, interventions have focused on its prevention and minimizing adverse effects of associated complications [1,4]. Through transfusing women within the reproductive ages with Kell-negative donor blood, if possible, and through the introduction of Rhesus (Rh) immunoglobulin prophylaxis, the occurrence of red blood cell alloimmunization and the prevalence of Rh(D)-and K-mediated HDFN has decreased [1,[4][5][6]; however, the gap between anti-Rh(D) supply and demand is large in low-income countries and is below the optimal threshold in high-income countries [7]. Additionally, the disease still poses a significant risk for mortality and morbidity in developing countries, whereas it is considered treatable with good outcomes in developed countries. Serological monitoring, ultrasonography, and Doppler imaging decreased the need for risky and invasive diagnostic procedures [3,[8][9][10][11][12]. Antenatal treatment, however, still relies predominantly on (often serial) intrauterine transfusion (IUT)-an invasive procedure that carries maternal and fetal risks [13,14].
Considering improvements in HDFN care, the objectives of this systematic literature review were to assess the prenatal treatment landscape and outcomes of Rh(D)-and K-mediated HDFN in mothers and fetuses to identify the burden of disease, to identify evidence gaps in the literature, and to provide recommendations for future research. Secondarily, we aim to determine the humanistic and economic burden of HDFN.

Search strategy
We conducted a systematic literature review according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [15] and MOOSE Reporting Guidelines for Meta-Analysis of Observational Studies [16] to address prespecified research questions (Table S2). To assess the treatment landscape, articles published between January 1, 2005, and March 10, 2021 were searched (Additional file 2: Appendix S1) in the MEDLINE and EMBASE databases and ClinicalTrials.gov using ProQuest (Fig. 1). The search strategy included descriptions of the disease, possible interventions and clinical outcomes. No limitations were set on studies reporting on cases managed before January 1, 2005. Searches for clinical outcomes were performed for journal articles and conference abstracts indexed in EMBASE. Duplicates were removed automatically. We also manually searched reference lists of pertinent systematic literature reviews of cohort studies and our personal libraries for potentially relevant articles.

Study selection
Two independent reviewers (D.P.D.W. and A.K.) (Table  S3) [17] reviewed the titles/abstracts in Rayyan (https:// rayyan. ai/) and then full texts in Microsoft Excel. Citations were independently evaluated to determine whether or not studies fulfilled inclusion and exclusion criteria. The project director (D.O.) and the project team adjudicated decisions. Randomized or nonrandomized trials; retrospective or prospective observational studies, including cohort, case-control, or cross-sectional studies; modelling studies; systematic reviews of cohort studies (to identify primary studies only); and case reports and case series of women and/or their fetuses, infants, or children experiencing or having experienced Rh(D)-and/ or K-mediated HDFN were included. Studies or patient groups within studies where HDFN was caused by alloimmunization to antigens other than Rh(D) only and/or K only, such as c, e, E, Duffy (Fy), Kidd (Jk), MNS (S), or Gerbich, were excluded as the risk of prenatal disease is regarded as relatively low. Non-English-language articles were excluded, as were notes, editorials, and commentaries; nonsystematic reviews; reports of populations, interventions, outcomes, or study designs not of interest; publication types not of interest; indexed conference abstracts; and reports of animal or preclinical studies. The review was registered with PROSPERO before data were abstracted.
Two independent reviewers (D.P.D.W. and A.K.) abstracted data (i.e., study reference; study design; patient characteristics; HDFN treatment patterns; clinical outcomes [eg, fetal anemia, hydrops fetalis, and adverse events]; intravenous immunoglobulin [IVIG] efficacy; mortality; and prevalence) from studies that fulfilled inclusion and exclusion criteria. All abstracted data underwent quality control by the project director (D.O.), who screened 10% of included/excluded articles. The Data from eligible studies were characterized as representative, which included data from studies that accurately reflected the characteristics of the larger group (e.g., larger case series, retrospective or prospective studies, RCTs), or were characterized as nonrepresentative, which included data from studies that reflected a small proportion of the characteristics of the larger group (e.g., case reports or small case series, or studies in a subset of the larger group, such as cases treated with IUT or only cases with hydrops fetalis). When possible, we aggregated information reported in a similar manner. For unique outcomes, we highlighted information from generalizable studies. Where appropriate, data were summarized as percentage (mean ± standard deviation [SD]  An assessment of the available findings was conducted to identify evidence gaps, and recommendations to fill unmet needs were formulated. Results pertinent to mothers and fetuses are reported herein. Neonatal outcomes will be reported in a separate article.

Humanistic and economic burden
A separate objective of this systematic review is to determine the humanistic and economic burden of HDFN. We conducted a systematic search using the same criteria as previously mentioned (Additional file 2: Appendix S2). We selected studies reporting on quality of life, humanistic burden, economic burden, health care resource use, and direct and indirect costs. The review process was performed according to the PRISMA and MOOSE guidelines and similar as previously stated.

Data sources
In addition to the 2,538 articles identified through searches of MEDLINE and EMBASE, we identified 3 articles from our personal libraries (Fig. 1). The search on ClinicalTrials.gov did not yield any additional results to the search on MEDLINE and EMBASE. Overall, 2,363 of the 2,541 total articles were excluded on the basis of title and abstract review, and 119 were excluded on the basis of full-text review. Besides the 59 articles that remained, we identified 1 article from a review of applicable systematic reviews of cohort studies.

Study characteristics
Among the 60 eligible studies that were included in our analysis (Table S1), [2,

Prevalence of D-and K-mediated HDFN
The mean ± SD prevalence of Rh(D)-mediated HDFN (requiring any form of treatment) as reported in 5 studies [23,29,45,47,54] was 0.047%±0.037% among all pregnancies that were managed and delivered in the centers of the 5 selected studies (Fig. 2). The reported prevalence of K-mediated HDFN (requiring any form of treatment) among all pregnancies managed and delivered in the centers reporting in 2 retrospective studies was 0.006% [45,47]. No data were available for the prevalence of early-onset HDFN (requiring intervention before 24 weeks of pregnancy) in the selected studies. The gestational age at first IUT was between 25 and 27 weeksand the mean gestational age at birth between 34 and 36 weeks (Table 1) amongst all selected studies [2,.

Intrauterine transfusions
Among the 9 studies with representative data, IUTs were given at a median frequency of 13.0% (IQR: 7.2-66.0) among pregnancies with a positive anti-Rh(D) screening that were monitored prenatally with ultrasonography. Three of these studies monitored pregnancies with a positive anti-Rh(D) screening and a risk-stratification using the antibody titers and/or antibody-dependent cellular cytotoxicity values above cut-off value, and might therefore overestimate the frequency of IUTs [56,59,60]. In these three studies, the frequency of IUTs was 64.7% (range 59.2-66) [56,59,60]. Number of IUTs was reported by only two of these studies with a median of 2 (range 0-4) [56,59]. IUTs were required in 76.8% of pregnancies, as reported in 63/82 collective cases [36,56,60]. The frequency of IUTs in pregnancies with a positive anti-Rh(D) screening monitored without serological cut-off values was 11.2% (range, 4.5-58.6) in the collective cases (42/376) reported by 5 studies [25,29,33,37,65]. The number of IUTs required was only reported by 1 study with a mean of 2.4 (SD not reported) [33]. Data on the need for IUTs in pregnancies with K-alloimmunization without serological cut-offs were reported by 1 study and was 12.5% in 1/8 reported cases [25].

Alternative management strategies
Use of IVIG alone was reported in 1 study with representative data [48]. In this case series of 3 severely affected pregnancies, 2 (Rh[D], n = 1; Kell, n = 1) resulted in live births without IUT; the third (Rh[D] + anti-C) was treated with IUT but resulted in a post-procedure intrauterine death.
Use of other treatments (therapeutic plasma exchange [TPE]; maternal plasma exchange ± highdose IVIG; TPE + IVIG + IUT; TPE + immunoadsorption + IVIG + IUT; plasmapheresis + IUT; and plasmapheresis + IVIG + IUT) were reported in 10 studies with a total of 38 cases [24,30,34,36,43,51,52,63,71,80]. Plasmapheresis + IVIG + IUT was the most commonly reported treatment regimen across these 10 studies.Two of these studies did not report gestational age at start of the treatment, at first IUT (if applicable) and at birth [36,80]. The remaining 8 studies, including 20 cases total, reported the mean gestational age at treatment initiation (13.0 ± 5.7 weeks). 17/20 cases required an IUT for fetal anemia. The mean gestational age at first IUT was 24.2 ± 3.1 weeks, with a median of 4 IUTs (range 1-8) administered. Gestational age at birth was 34.4 ± 3.1 weeks [30,34,43,51,52,63,71]. In the series of 20 cases, one patient received plasmapheresis, which was started a HDFN Hemolytic disease of the fetus and newborn, IUT Intrauterine transfusion a Of the 155 patient groups, 46 (29.7%) groups were single patients from case reports b Does not include 1 study [2] in which the reported percentage of the patient group fell within gestational age ranges (i.e., < 259 days and 259-294 days) week after the first IUT at a gestational age of 27 weeks [43]. In all other cases, the alternative treatment was started prior to the occurrence of fetal anemia. The indications to start the alternative treatment option in the 20 cases were previous intrauterine fetal death (n = 11), neonatal hydrops fetalis and/or death (n = 4), marked elevation in antibody titer (n = 4), and suspected fetal anemia after initial IUT (n = 1).

Fetal mortality
The overall mean ± SD fetal mortality rate was 19.8% ± 29.4% across 19 studies, including representative case reports [28,31,35,42,43,45,47,49,53,63,64,66,68,69,73,75,76,78,80] (Table S8). Head-to-head comparison of mortality rates between the different treatment strategies is limited by variation in potential patient characteristics between the groups. Employed treatment strategies for HDFN take into account previous obstetrical history, for example 75% of cases in the "IUT + other" group had a history of fetal or neonatal death due to HDFN. Together, these will influence the outcomes of HDFN in the current pregnancy and limit our capability of mortality rate comparison.

Humanistic and economic burden
In addition to the 1457 articles identified in the systematic search, one additional article was identified from personal libraries (Fig. S2). Based on the title/abstract screening 1435 articles were excluded. Full-text screening was performed in the remaining 23 records of which 22 were excluded. Healthcare utilization was reported by only one study with a median of duration of phototherapy of 4-4.5 days and a median length of stay of 6.5-7.5 days [65].

Main findings
We found that the prenatal burden and need for treatment remains relatively high -we estimated that 13% of pregnancies monitored for Rh(D) or K-alloimmunization required one or more IUTs -despite advances in the identification and care for pregnancies at risk of HDFN. Strikingly, the rate of hydrops fetalis in pregnancies requiring an IUT was found to be 14.9% for Rh(D)-mediated HDFN and 39.2% in K-mediated HDFN. As the occurrence of hydrops fetalis was previously found to be associated with impaired neurodevelopmental outcomes [81] still much is to gain in the timely identification of pregnancies at-risk and the timely detection and treatment of fetal anemia to prevent hydrops fetalis. The average gestational age at first IUT was 27 weeks, which was possibly delayed by using IVIG and/or plasmapheresis although evidence on this in the included studies is limited. Although IUT is a regarded as a relatively safe procedure in experienced hands, its invasive nature still poses serious risks to the mother and fetus. Fetal loss rate increases when procedures need to be done early in gestation (i.e., < 22 weeks) [13]. It is also noteworthy that the average gestational age at birth in the present analyses was approximately 35 weeks for Rh(D)-and/ or K-mediated HDFN, which is considered late preterm and might also represent that early delivery is frequently employed in the management of pregnancies at risk of fetal anemia although we were unable to extract data on this from the included studies. But, late preterm birth has the potential for serious consequences, such as increased risk for short-or long-term respiratory issues [82][83][84], readmission [82], death [82,84,85], and neurocognitive impairment in late adulthood [86,87].

Strengths and limitations
A strength of this systematic review and corresponding analyses is the minimal limitation on study design criteria. Overall, 35% of the studies included in our analyses were case reports or case series, validating the rarity of HDFN. By including case reports and case series, we were able to identify and aggregate data on treatment types (e.g., plasmapheresis and plasma exchange) that were not typically reported in larger cohort studies. But, inclusion of case reports and case series might also be regarded as a limitation as it may skew and overestimate the results as, given the rarity of HDFN, the most severe cases are generally reported in literature. Our estimates might therefore not truly mirror the population level data. Also, we were unable to quantify heterogeneity (using e.g. the I 2 -statistic) due to the descriptive nature of this systematic review and consequent lack of reported comparisons between interventions. However, a certain level of heterogeneity may be expected due to differences in available management options, treatment protocols, prevalence of Rh(D)-and K-negativity, geographical location and sociodemographic differences. These differences may be approached by the varying frequencies of, for instance, IUTs and rate of hydrops fetalis between included studies.
Our analyses are further limited by the strict prespecified inclusion of only Rh(D)-or K-mediated HDFN populations. By applying this criterion, 7 studies were excluded from analyses-despite high quality of evidence and outcomes of interest-because Rh(c) populations were mixed with Rh(D) or Kell populations, or the population was not well defined [88][89][90][91][92][93][94]. We were unable to stratify the data per alloimmunization type with the data provided in these articles. One of these studies represented the only prospective study with long-term outcomes [89], thereby also indicating the lack of data on long-term outcomes and the need for further research on the topic.

Interpretation
Almost all studies included in these analyses were conducted in high-income countries, which have adequate resources for screening, prophylaxis and preventative measures for alloimmunization, and referral to specialized fetal therapy centers. Outcome data on HDFNcomplicated pregnancies from less privileged or less organized societies are lacking and, if analyzed, likely are less favorable. This well-known bias in outcome reporting indicates an important evidence gap and signifies the need for international collaboration to gain a better understanding of the global burden of HDFN and to pave the way for potential wide-spread improvements. To add to that, we also found that the evidence for frequency of use and effectiveness of alternative treatment options such as IVIG, plasmapheresis, and plasma exchange on disease severity and the prevention of fetal anemia is limited in the included studies. Also, as previously mentioned it is likely that the most severe cases are reported in literature due to the rarity of the disease. Taken together, future research should aim to gain more exact insight into the employed treatment options and its efficacy and clinical outcomes of mothers, fetuses, and neonates affected by HDFN through an international retrospective and/or prospective registry through the collection of data on diagnostics, antenatal and postnatal treatments and short-and long-term clinical outcomes of mothers, fetuses, and neonates. Such an international effort will pave the way for long sought after answers.
A separate objective of this systematic review, as previously mentioned, was to ascertain the economic and humanistic burden of HDFN. However, through the systematic approach only one study reporting on healthcare utilization was included. This dearth of information indicates another major gap in knowledge, particularly as it relates to the impact of HDFN on a pregnant individual's quality of life and the potential downstream consequences of high-risk pregnancy on family planning decisions, as well as on the healthcare system.

Conclusion
To conclude, we found that the clinical burden of Rh(D)and K-mediated HDFN remains relatively high, with 13% of pregnancies monitored for Rh(D)-or K-alloimmunization requiring an IUT and most births occurring at a late preterm gestational age. We identified several important evidence gaps that provide opportunities for future studies to further improve the clinical care of HDFN.
Additional file 1: Table S1. General Characteristics of Studies and Patient Groups.